Integrated cellular communications system

ABSTRACT

A cellular communications system is provided having both surface and satellite nodes which are fully integrated for providing service over large areas. A spread spectrum system is used with code division multiple access (CDMA) employing forward error correction coding (FECC) to enhance the effective gain and selectivity of the system. Multiple beam, relatively high gain antennas are disposed in the satellite nodes to establish a first set of cells, and by coupling the extra gain obtained with FECC to the high gain satellite-node antennas, enough gain is created in the satellite part of the system such that a user need only use a small, mobile handset with a non-directional antenna for communications with both ground nodes and satellite nodes. User position information is also available. A digital data interleaving feature reduces fading. Each transmitter includes a coder representative of its output power level. Receivers compare this code to the received signal strength and adjust their associated transmitter power output level accordingly. An inter-cell bus is formed in a super-surface node which receives signals from a plurality of cells, frequency multiplexes those signals and transmits the multiplexed signal to the plurality of cells.

This application is a continuation-in-part of application Ser. No.07/495,497 filed Mar. 19, 1990, now U.S. Pat. No. 5,073,900.

BACKGROUND

The invention relates to communication systems and in particular, to acellular mobile communications system having integrated satellite andground nodes.

The cellular communications industry has grown at a fast pace in theUnited States and even faster in some other countries. It has become animportant service of substantial utility and because of the growth rate,saturation of the existing service is of concern. High density regionshaving high use rates, such as Los Angeles, New York and Chicago are ofmost immediate concern. Contributing to this concern is the congestionof the electromagnetic frequency spectrum which is becoming increasinglysevere as the communication needs of society expand. This congestion iscaused not only by cellular communications systems but also by othercommunications systems. However, in the cellular communications industryalone, it is estimated that the number of mobile subscribers willincrease on a world-wide level by an order of magnitude within the nextten years. The radio frequency spectrum is limited and in view of thisincreasing demand for its use, means to more efficiently use it arecontinually being explored.

Existing cellular radio is primarily aimed at providing mobile telephoneservice to automotive users in developed metropolitan areas. For remotearea users, airborne users, and marine users, AIRFONE and INMARSATservices exist but coverage is incomplete and service is relativelyexpensive. Mobile radio satellite systems in an advanced planning stagewill probably provide improved direct-broadcast voice channels to mobilesubscribers in remote areas but still at significantly higher cost incomparison to existing ground cellular service. The ground cellular andplanned satellite technologies complement one another in geographicalcoverage in that the ground cellular communications service providesvoice telephone service in relatively developed urban and suburban areasbut not in sparsely populated areas, while the planned earth orbitingsatellites will serve the sparsely populated areas. Although the twotechnologies use the same general area of the RF spectrum, they arebasically separate and incompatible by design as they presently exist.At present, if a user needs both forms of mobile communicationscoverage, he must invest in two relatively expensive subscriber units,one for each system.

The demand for mobile telephone service is steadily expanding and withthe expansion of the service, the problem of serving an increased numberof subscribers who are travelling from one region to another has becomeof primary importance. Cellular communications systems divide theservice areas into geographical cells, each served by a base station ornode typically located at its center. The central node transmitssufficient power to cover its cell area with adequate field strength. Ifa mobile user moves to a new cell, the radio link is switched to the newnode provided there is an available channel. However, if the mobile usertravels into a region where all channels are busy, or that is not servedby any cellular service, or, in some cases, into an area served by adifferent licensee/provider, then his call may be abruptly terminated.

Present land mobile communication systems typically use a frequencymodulation (FM) approach and because of the limited interferencerejection capabilities of FM modulation, each radio channel may be usedonly once over a wide geographical area encompassing many cells. Thismeans that each cell can use only a small fraction of the totalallocated radio frequency band, resulting in an inefficient use of theavailable spectrum. In some cases, the quality of speech is poor becauseof the phenomena affecting FM transmission known as fading and "deadspots." The subjective effect of fading is repeated submersion of thevoice signal in background noise frequently many times per second if themobile unit is in motion. The problem is exacerbated by interferencefrom co-channel users in distant cells and resultant crosstalk due tothe limited interference rejection capability of FM. Additionally,communications privacy is relatively poor; the FM signal may be heard byothers who are receiving that frequency.

In the case where one band of frequencies is preferable over others andthat one band alone is to be used for mobile communications, efficientcommunications systems are necessary to assure that the number of usersdesiring to use the band can be accommodated. For example, there ispresently widespread agreement on the choice of L-band as thetechnically preferred frequency band for the satellite-to-mobile link inmobile communications systems. In the case where this single band ischosen to contain all mobile communications users, improvements inspectral utilization in the area of interference protection and in theability to function without imposing intolerable interference on otherservices will be of paramount importance in the considerations ofoptimal use of the scarce spectrum.

The spread spectrum communications technique is a technology that hasfound widespread use in military applications which must meetrequirements for security, minimized likelihood of signal detection, andminimum susceptibility to external interference or jamming. In a spreadspectrum system, the data modulated carrier signal is further modulatedby a relatively wide-band, pseudo-random "spreading" signal so that thetransmitted bandwidth is much greater than the bandwidth or rate of theinformation to be transmitted. Commonly the "spreading" signal isgenerated by a pseudo-random deterministic digital logic algorithm whichis duplicated at the receiver.

By further modulating the received signal by the same spreadingwaveform, the received signal is remapped into the original informationbandwidth to reproduce the desired signal. Because a receiver isresponsive only to a signal that was spread using the same uniquespreading code, a uniquely addressable channel is possible. Also, thepower spectral density is low and without the unique spreading code, thesignal is very difficult to detect, much less decode, so privacy isenhanced and interference with the signals of other services is reduced.The spread spectrum signal has strong immunity to multipath fading,interference from other users of the same system, and interference fromother systems.

In a satellite communications system, downlink power is an importantconsideration. Satellite power is severely limited; therefore, thenumber of users of the satellite that can be accommodated, andconsequently the economic viability of such a system, is in inverseproportion to how much satellite transmitter power must be allocated toeach user. Many of the proposed mobile communications satellite systemshave relied upon user antenna directivity to provide additionaleffective power gain. This has resulted in significant user equipmentexpense and the operational inconvenience of having to perform somesteering or selection of the antenna to point at the satellite.Additionally, hand held transceivers are impractical because of therelatively large directive antennas required.

In some ground cellular service, the user transceiver commonly radiatesat a power level which is 30 to 40 dB greater than is required on theaverage in order to overcome fading nulls. This results in greatlyincreased inter-system interference and reduced battery life. It wouldalso be desirable to provide a power control system to compensate forfading and interference without exceeding the minimum amount of powernecessary to overcome such interference.

Additionally, a user position determination capability would be usefulfor certain applications of a cellular communications system such astracking the progress of commercial vehicles en route. A further use maybe to provide users with an indication of their own position. Such acapability would be more useful with increased accuracy.

Thus it would be desirable to provide a cellular communications systemwhich integrates satellite nodes with surface nodes to provide coverageof greater surface areas without requiring the use of two differentsystems with attendant expense and hardware requirements. Additionally,it would be desirable to provide a cellular communications system usinga spread spectrum technique which can make more efficient use ofexisting frequency spectrum resources and result in increased privacy incommunications. Additionally, it would be desirable to permit the use ofa relatively low power, compact and mobile user handset having a small,non-directional antenna, one which can communicate with both theland-based stations and the satellite-based stations.

SUMMARY OF THE INVENTION

The invention provides a cellular communications system having bothsurface and space nodes which are fully integrated. Areas where surfacenodes are impractical are covered by a space node. Space nodes comprisesatellites which establish cells which in many cases overlap groundcells. A spread spectrum communications method is used which includescode division multiple access (CDMA) and forward error correction coding(FECC) techniques to increase the number of users that can beaccommodated within the allocated spectrum. The spread spectrum systemmakes possible the use of very low rate, highly redundant coding withoutloss of capability to accommodate the largest possible number of userswithin the allocated bandwidth. The low rate coding in turn providesmaximum possible coding gain, minimizing the required signal strength atthe receiver and maximizing the number of users that can be served in agiven frequency band.

Relatively high gain, multiple-beam antennas are used on the satellitesand in one embodiment, antennas having a relatively large reflector witha multiple element feed positioned in the focal plane of the reflectorare used. By coupling a high gain antenna with the extra gain obtainedwith FECC, enough gain exists in the system such that the user unitcomprises only a small, mobile handset with a small, non-directionalantenna.

An adaptive transmitter power control system compensates for receivedsignal strength variations, such as those caused by buildings, foliageand other obstructions. A path loss estimate is derived from thereceived signal strength and from data included in each transmittedsignal which indicates that transmitter's output power. Based on thederived path loss and the transmitter's power level data, the receivercan then adjust the power output of its own associated transmitteraccordingly.

In one embodiment, a system network control center is used to coordinatesystem-wide operations, to keep track of user locations, to performoptimum allocation of system resources to each call, dispatch facilitycommand codes, and monitor and supervise overall system health. Overallsystem control is of a hierarchical nature in this embodiment comprisingthe system network control center, regional node control centers whichcoordinate the detailed allocation of ground network resources within aregion, and one or more satellite node control centers responsible forallocation of resources among the satellite network resources. Inanother embodiment, the system does not include a system network controlcenter and the node control centers operate autonomously.

In one embodiment, one or more satellite node control centers serve amultiplicity, M, of satellite cells comprising a "cluster." In thisembodiment, the M composite signals to and from the various clustermember cells are frequency multiplexed onto the common backhaul link,and are separated by frequency demultiplex at the one or more satellitenode control centers serving the cluster. In this embodiment, the numberM of cells in the cluster is a design variable which can range betweenone and the total number of cells in the system. This can be optimizedfor each particular cluster region depending upon available backhaulmultiplex bandwidth and local telephone company intra-regional callrates.

In another aspect of the invention, an inter-cell bus system is providedin which a user's uplink communication with a satellite in one cell maybe simultaneously downlinked to all cells of the same satellite on thebus.

In yet another aspect of the invention, position determination of a useris provided by monitoring the user response signal to a polling or othersignal transmitted by the position locating equipment. Time differencesof arrival at several nodes provide the data basis for determining thelocation of the particular user.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an overview of the principal elementsof a communications system in accordance with the principles of theinvention;

FIG. 2 is a diagram of the frequency sub-bands of the frequency bandallocation for a cellular system;

FIG. 3 is a overview block diagram of a communications system inaccordance with the principles of the invention without a networkcontrol center;

FIG. 4 is a diagram showing the interrelationship of the cellularhierarchical structure of the ground and satellite nodes in a typicalsection and presents a cluster comprising more than one satellite cell;

FIG. 5 is a block diagram of a satellite link system showing the userunit and satellite node control center;

FIG. 6 is a block diagram of one embodiment of satellite signalprocessing in the system of FIG. 5;

FIG. 7 is a functional block diagram of a user transceiver showing anadaptive power control system; and

FIGS. 8a through 8h show timing diagrams of an adaptive, two-way powercontrol system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is shown in the exemplary drawings, the invention is embodied in acellular communications system utilizing integrated satellite and groundnodes both of which use the same modulation, coding, and spreadingstructure and both responding to an identical user unit.

Referring now to FIG. 1, an overview of a communications system 10 ispresented showing the functional inter-relationships of the majorelements. The system network control center 12 directs the top levelallocation of calls to satellite and ground regional resourcesthroughout the system. It also is used to coordinate system-wideoperations, to keep track of user locations, to perform optimumallocation of system resources to each call, dispatch facility commandcodes, and monitor and supervise overall system health. The regionalnode control centers 14, one of which is shown, are connected to thesystem network control center 12 and direct the allocation of calls toground nodes within a major metropolitan region. The regional nodecontrol center 14 provides access to and from fixed land communicationlines, such as commercial telephone systems known as the public switchedtelephone network (PSTN). The ground nodes 16 under direction of therespective regional node control center 14 receive calls over the fixedland line network encode them, spread them according to the uniquespreading code assigned to each designated user, combine them into acomposite signal, modulate that composite signal onto the transmissioncarrier, and broadcast them over the cellular region covered.

Satellite node control centers 18 are also connected to the systemnetwork control center 12 via status and control land lines or othermeans and similarly handle calls designated for satellite links such asfrom the PSTN, encode them, spread them according to the uniquespreading codes assigned to the designated users, and multiplex themwith other similarly directed calls into an uplink trunk, which isbeamed up to the designated satellite 20. Satellite nodes 20 receive theuplink trunks, frequency demultiplex the calls intended for differentsatellite cells, frequency translate and direct each to its appropriatecell transmitter and cell beam, and broadcast the composite of all suchsimilarly directed calls down to the intended satellite cellular area.As used herein, "backhaul" means the link between a satellite 20 and asatellite node control center 18. In one embodiment, it is a K-bandfrequency while the link between the satellite 20 and the user unit 22uses an L-band or an S-band frequency.

User units 22 respond to signals of either satellite or ground nodeorigin, receive the outbound composite signal, separate out the signalintended for that user by despreading using the user's assigned uniquespreading code, de-modulate, and decode the information and deliver thecall to the user. Such user units 22 may be mobile or may be fixed inposition. Gateways 24 provide direct trunks, that is, groups ofchannels, between satellite and the ground public switched telephonesystem or private trunk users. For example, a gateway may comprise adedicated satellite terminal for use by a large company or other entity.In the embodiment of FIG. 1, the gateway 24 is also connected to thatsystem network controller 12.

All of the above-discussed centers, nodes, units and gateways are fullduplex transmit/receive performing the corresponding inbound (user tosystem) link functions as well in the inverse manner to the outbound(system to user) link functions just described.

Referring now to FIG. 2, the allocated frequency band 26 of acommunications system is shown. The allocated frequency band 26 isdivided into 2 main sub-bands, an outgoing sub-band 25 and an incomingsub-band 27. Additionally the main sub-bands are themselves divided intofurther sub-bands which are designated as follows:

OG: Outbound Ground 28 (ground node to user)

OS: Outbound Satellite 30 (satellite node to user)

OC: Outbound Calling and Command 32 (node to user)

IG: Inbound Ground 34 (user to ground node)

IS: Inbound Satellite 36 (user to satellite node)

IC: Inbound Calling and Tracking 38 (user to node)

All users in all cells use the entire designated sub-band for thedescribed function. Unlike existing ground or satellite mobile systems,there is no necessity for frequency division by cells; all cells may usethese same basic six sub-bands. This arrangement results in a higherfrequency reuse factor as is discussed in more detail below.

In one embodiment, a mobile user's unit 22 will send an occasional burstof an identification signal in the IC sub-band either in response to apoll or autonomously. This may occur when the unit 22 is in standbymode. This identification signal is tracked by the regional node controlcenter 14 as long as the unit is within that respective region,otherwise the signal will be tracked by the satellite node or nodes. Inanother embodiment, this identification signal is tracked by all groundand satellite nodes capable of receiving it. This information isforwarded to the network control center 12 via status and command linesor other means. By this means, the applicable regional node controlcenter 14 and the system network control center 12 remain constantlyaware of the cellular location and link options for each active user 22.An intra-regional call to or from a mobile user 22 will generally behandled solely by the respective regional node control center 14.Inter-regional calls are assigned to satellite or ground regional systemresources by the system network control center 12 based on the locationof the parties to the call, signal quality on the various link options,resource availability and best utilization of resources.

A user 22 in standby mode constantly monitors the common outboundcalling frequency sub-band OC 32 for calling signals addressed to him bymeans of his unique spreading code. Such calls may be originated fromeither ground or satellite nodes. Recognition of his unique call codeinitiates the user unit 22 ring function. When the user goes "off-hook",e.g. by lifting the handset from its cradle, a return signal isbroadcast from the user unit 22 to any receiving node in the usercalling frequency sub-band IC 38. This initiates a handshaking sequencebetween the calling node and the user unit which instructs the user unitwhether to transition to either satellite, or ground frequencysub-bands, OS 30 and IS 36 or OG 28 and IG 34.

A mobile user wishing to place a call simply takes his unit 22 off hookand dials the number of the desired party, confirms the number and"sends" the call. Thereby an incoming call sequence is initiated in theIC sub-band 38. This call is generally heard by several ground andsatellite nodes which forward call and signal quality reports to theappropriate system network control center 12 which in turn designatesthe call handling to a particular satellite node 20/satellite nodecontrol center 18 or regional node control center 14 or both. The callhandling element then initiates a handshaking function with the callingunit over the OC 32 and IC 38 sub-bands, leading finally to transitionto the appropriate satellite or ground sub-bands for communication.

Referring now to FIG. 3, a block diagram of a communications system 40which does not include a system network control center is presented. Inthis system, the satellite node control centers 42 are connecteddirectly into the land line network as are also the regional nodecontrol centers 44. Gateway systems 46 are also available as in thesystem of FIG. 1. and connect the satellite communications to theappropriate land line or other communications systems. The user unit 22designates satellite node 20 communication or ground node 50communication by sending a predetermined code.

Referring now to FIG. 4, a hierarchical cellular structure is shown. Apair of clusters 52 of ground cells 54 are shown. Additionally, aplurality of satellite cells 56 are shown. Although numerals 54 and 56point only to two cells each, this has been done to retain clarity inthe drawing. Numeral 54 is meant to indicate all ground cells in thefigure and similarly numeral 56 is meant to indicate all satellitecells. The cells are shown as hexagonal in shape, however, this isexemplary only. The ground cells may be from 3 to 15 km across althoughother sizes are possible depending on user density in the cell. Thesatellite cells may be approximately 200-500 km across as an exampledepending on the number of beams used to cover a given area. As shown,some satellite cells may include no ground cells. Such cells may coverundeveloped areas for which ground nodes are not practical. Part of asatellite cluster 58 is also shown. The cell members of such a clustershare a common satellite node control center 60.

A significant advantage of the invention is that by the use of spreadspectrum multiple access, adjacent cells are not required to usedifferent frequency bands. All ground-user links utilize the same twofrequency sub-bands (OG 28, IG 34) and all satellite-user links use thesame two frequency sub-bands (OS 30, IS 36). This obviates an otherwisecomplex and restrictive frequency coordination problem of ensuring thatfrequencies are not reused within cells closer than some minimumdistance to one another (as in the FM approach), and yet provides for ahierarchical set of cell sizes to accommodate areas of significantlydifferent subscriber densities.

Referring again to FIG. 1 as well as to FIG. 4, the satellite nodes 20make use of large, multiple-feed antennas 62 which in one embodimentprovide separate beams and associated separate transmitters for eachsatellite cell 56. For example, the multiple feed antenna 62 may coveran area such as the United States with, typically, about 100 satellitebeams/cells and in one embodiment, with about 200 beams/cells. Thecombined satellite/ground nodes system provides a hierarchicalgeographical cellular structure. Thus within a dense metropolitan area,each satellite cell 56 may further contain as many as 100 or more groundcells 54, which ground cells would normally carry the bulk of thetraffic originated therein. The number of users of the ground nodes 16is anticipated to exceed the number of users of the satellite nodes 20where ground cells exist within satellite cells. Because all of theseground node users would otherwise interfere as background noise with theintended user-satellite links, in one embodiment the frequency bandallocation may be separated into separate segments for the groundelement and the space element as has been discussed in connection withFIG. 2. This combined, hybrid service can be provided in a manner thatis smoothly transparent to the user. Calls will be allocated among allavailable ground and satellite resources in the most efficient manner bythe system network control center 12.

An important parameter in most considerations of cellular radiocommunications systems is the "cluster", defined as the minimal set ofcells such that mutual interference between cells reusing a givenfrequency sub-band is tolerable provided that such "co-channel cells"are in different clusters. Conversely all cells within a cluster mustuse different frequency sub-bands. The number of cells in such a clusteris called the "cluster size". It will be seen that the "frequency reusefactor", i.e., the number of possible reuses of a frequency sub-bandwithin the system is thus equal to the number of cells in the systemdivided by the cluster size. The total number of channels that can besupported per cell, and therefore overall bandwidth efficiency of thesystem is thus inversely proportional to the cluster size. By means tobe described, the invention system achieves a minimum possible clustersize of one as compared to typically 7 to 13 for other ground orsatellite cellular concepts and thereby a maximum possible frequencyreuse factor. This is a major advantage of the invention.

Referring now to FIG. 5, a block diagram is shown of a typical user unit22 to satellite 20 to satellite node control 18 communication and theprocessing involved in the user unit 22 and the satellite node control18. In placing a call for example, the handset 64 is lifted and thetelephone number entered by the user. After confirming a display of thenumber dialed, the user pushes a "send" button, thus initiating a callrequest signal. This signal is processed through the transmitterprocessing circuitry 66 which includes spreading the signal using acalling spread code. The signal is radiated by the omni-directionalantenna 68 and received by the satellite 20 through its narrow beamwidthantenna 62. The satellite processes the received signal as will bedescribed below and sends the backhaul to the satellite node controlcenter 18 by way of its backhaul antenna 70. On receive, the antenna 68of the user unit 22 receives the signal and the receiver processor 72processes the signal. Processing by the user unit 22 will be describedin more detail below in reference to FIG. 7.

The satellite node control center 18 receives the signal at its antenna71, applies it to a circulator 73, amplifies 74, frequency demultiplexes76 the signal separating off the composite signal which includes thesignal from the user shown in FIG. 5, splits it 78 off to one of a bankof code correletors, each of which comprises a mixer 80 for removing thespreading and identification codes, an AGC amplifier 82, the FEC decoder84, a demultiplexer 86 and finally a voice encoder/decoder (CODEC) 88for converting digital voice information into an analog voice signal.The voice signal is then routed to the appropriate land line, such as acommercial telephone system. Transmission by the satellite node controlcenter 18 is essentially the reverse of the above described receptionoperation.

Referring now to FIG. 6, the satellite transponder 90 of FIG. 5 is shownin block diagram form. A circulator/diplexer 92 receives the uplinksignal and applies it to an L-band or S-band amplifier 94 asappropriate. The signals from all the M satellite cells within a"cluster" are frequency multiplexed 96 into a single composite K-bandbackhaul signal occupying M times the bandwidth of an individualL-/S-band mobile link channel. The composite signal is then split 98into N parts, separately amplified 100, and beamed through a secondcirculator 102 to N separate satellite ground cells. This generalconfiguration supports a number of particular configurations various ofwhich may be best adapted to one or another situation depending onsystem optimization which for example may include considerations relatedto regional land line long distance rate structure, frequency allocationand subscriber population. Thus, for a low density rural area, one mayutilize an M-to-1 (M>1, N=1) cluster configuration of M contiguous cellsserved by a single common satellite ground node with M limited byavailable bandwidth. In order to provide high-value, long distanceservice between metropolitan areas already or best covered for localcalling by ground cellular technology, an M-to-M configuration wouldprovide an "inter-metropolitan bus" which would tie together alloccupants of such M satellite cells as if in a single local callingregion. To illustrate, the same cells (for example, Seattle, LosAngeles, Omaha and others) comprising the cluster of M user cells on theleft side of FIG. 6, are each served by corresponding backhaul beams onthe right side of FIG. 6.

Referring now to FIG. 7, a functional block diagram of a typical userunit 22 is shown. The user unit 22 comprises a small, light-weight,low-cost, mobile transceiver handset with a small, non-directionalantenna 68. The single antenna 68 provides both transmit and receivefunctions by the use of a circulator/diplexer 104 or other means. It isfully portable and whether stationary or in motion, permits access to awide range of communication services from one telephone with one callnumber. It is anticipated that user units will transmit and receive onfrequencies in the 1-3 Ghz band but can operate in other bands as well.

The user unit 22 shown in FIG. 7 comprises a transmitter section 106 anda receiver section 108. For the transmission of voice communication, amicrophone couples the voice signal to a voice encoder 110 whichperforms analog to digital encoding using one of the various modernspeech coding technologies well known to those skilled in the art. Thedigital voice signal is combined with local status data, and/or otherdata, facsimile, or video data forming a composite bit stream in digitalmultiplexer 112. The resulting digital bit stream proceeds sequentiallythrough forward error encoder 114, symbol or bit interleaver 116, symbolor bit, phase, and/or amplitude modulator 118, narrow band IF amplifier120, wideband multiplier or spreader 122, wide band IF amplifier 124,wide band mixer 126, and final power amplifier 128. Oscillators orequivalent synthesizers derive the bit or baud frequency 130,pseudo-random noise or "chip" frequency 132, and carrier frequency 134.The PRN generator 136 comprises deterministic logic generating apseudo-random digital bit stream capable of being replicated at theremote receiver. The ring generator 138 on command generates a shortpseudo-random sequence functionally equivalent to a "ring."

The transceiver receive function 108 demodulation operations mirror thecorresponding transmit modulation functions in the transmitter section106. The signal is received by the non-directional antenna 68 andconducted to the circulator 104. An amplifier 142 amplifies the receivedsignal for mixing to an IF at mixer 144. The IF signal is amplified 146and multiplied or despread 148 and then IF amplified 150 again. The IFsignal then is conducted to a bit or symbol detector 152 which decidesthe polarity or value of each channel bit or symbol, a bit or symbolde-interleaver 154 and then to a forward error decoder 156. Thecomposite bit stream from the FEC decoder 156 is then split into itsseveral voice, data, and command components in the de-multiplexer 158.Finally a voice decoder 160 performs digital to analog converting andresults in a voice signal for communication to the user by a speaker orother means. Local oscillator 162 provides the first mixer 144 LO andthe bit or symbol detector 152 timing. A PRN oscillator 164 and PRNgenerator 166 provide the deterministic logic of the spread signal fordespreading purposes. The baud or bit clock oscillator 168 drives thebit in the bit detector 152, forward error decoder 156 and the voicedecoder 160.

The bit or symbol interleaver 116 and de-interleaver 154 provide a typeof coded time diversity reception which provides an effective power gainagainst multipath fading to be expected for mobile users. Its functionis to spread or diffuse the effect of short bursts of channel bit orsymbol errors so that they can more readily be corrected by the errorcorrection code.

As an alternative mode of operation, provision is made for direct dataor facsimile or other digital data input 170 to the transmitter chainand output 172 from the receiver chain.

A command decoder 174 and command logic element 176 are coupled to theforward error decoder 156 for receiving commands or information. Bymeans of special coding techniques known to those skilled in the art,the non-voice signal output at the forward error decoder 156 may beignored by the voice decoder 160 but used by the command decoder 174. Anexample of the special coding techniques are illustrated in FIG. 7 bythe MUX 112 and DEMUX 158.

As shown, acquisition, control and tracking circuitry 178 are providedin the receiver section 108 for the three receive side functionaloscillators 162, 164, 168 to acquire and track the phase of theircounterpart oscillators in the received signal. Means for so doing arewell known to those skilled in the art.

The automatic gain control (AGC) voltage 184 derived from the receivedsignal is used in the conventional way to control the gain of thepreceding amplifiers to an optimum value and in addition as an indicatorof short term variations of path loss suffered by the received signal.By means to be described more in detail below, this information iscombined with simultaneously received digital data 186 in a power levelcontroller 188 indicating the level at which the received signal wasoriginally transmitted to command the local instantaneous transmit powerlevel to a value such that the received value at the satellite nodecontrol is approximately constant, independent of fading and shadowingeffects. The level commanded to the output power amplifier 128 is alsoprovided 190 to the transmitter multiplexer 112 for transmission to thecorresponding unit.

In mobile and other radio applications, fading, shadowing, andinterference phenomena result in occasional, potentially significantsteep increases of path loss. In order to insure that the probabilitythat such a fade will be disruptive is acceptably low, conventionaldesign practice is to provide a substantial excess power margin bytransmitting a power which is normally as much as 10 to 40 dB above theaverage requirement. But this causes correspondingly increased batteryusage, inter-system, and intra-system interference. In a CDMAapplication, this can drastically reduce the useful circuit capacity ofthe channel.

A further feature of a system in accordance with the principles of theinvention is an adaptive control which continually maintains eachtransmitted signal power at a minimum necessary level, adapting rapidlyto and accommodating such fades dynamically, and only as necessary. Eachtransmitter telemeters its current signal output level to thecounterpart far end receiver by adding a low rate data stream to thecomposite digital output signal. Using this information along with themeasured strength of the received signal and assuming path lossreciprocity, each end can form an estimate of the instantaneous pathloss and adjust its current transmit power output to a level which willproduce an approximately constant received signal level at thecounterpart receiver irrespective of path loss variations.

Referring now to FIGS. 8a through 8h, timing and waveform diagrams of anadaptive power control system in accordance with the principles of theinvention are presented. In this example, the two ends of thecommunications link are referred to generally as A and B. In the groundcellular application, "A" corresponds to the user and "B" corresponds tothe cellular node. In the satellite link, A would be the user and Bwould be the satellite control node; in this case, the satellite issimply a constant gain repeater and the control of its power output isexercised by the level of the signal sent up to it.

In the example of FIG. 8a, at time 192, the path loss suddenly increasesx dB due for example to the mobile user A driving behind a building orother obstruction in the immediate vicinity of A. This causes the signalstrength as sensed by A's AGC to decrease x dB as shown in FIG. 8b. Thetelemetered data at time 192 shown in FIG. 8c indicates that the levelat which this signal has been transmitted from B had not been altered,A's power level controller 188 subtracts the telemetered transmittedsignal level from the observed received signal level and computes thatthere has been an increase of x dB in path loss. Accordingly itincreases its signal level output by x dB at time 192 as shown in FIG.8d and at the same time adds this information to its status telemeterchannel.

This signal is transmitted to B, arriving after transit time T as shownin FIG. 8e. The B receiver sees a constant received signal strength asshown in FIG. 8f but learns from the telemetered data channel as shownin FIG. 8g that the signal has been sent to him at +x dB. Therefore, Balso computes that the path loss has increased x dB, adjusts its outputsignal level accordingly at FIG. 8h and telemeters that information.That signal increase arrives back at station A at 2 T as shown in FIG.8e thus restoring the nominal signal strength with a delay of twotransit times (T). Thus for a path loss variation occurring in thevicinity of A, the path loss compensation at B is seen to be essentiallyinstantaneous while that at A occurs only after a two transit timedelay, 2 T.

Referring again to FIG. 7, an arrangement is provided for generatingcall requests and detecting ring signals. The ring generator 138generates a ring signal based on the user's code for calling out withthe user unit 22. For receiving a call, the ring signal is detected in afixed matched filter 198 matched to a short pulse sequence which carriesthe user's unique code. By this means each user can be selectivelycalled. As an option, the ring detect and call request signals may beutilized in poll/response mode to provide tracking information on eachactive or standby mode user. Course tracking information, adequate formanagement of the call routing functions is provided by comparison ofsignal quality as received at various modes. For the precision locationoption, the user response signal time is accurately locked to the timeof receipt of the timing (polling) signal which establishes a uniquelyidentifiable timing epoch, to a fraction of a PRN chip width.Measurement of the round trip poll/response time from two or more nodesor time differences of arrival at several nodes provides the basicmeasurement that enable solution and provision of precise user position.Ground and satellite transmitters and receivers duplicate the functionssummarized above for the user units. Given a priori information, asingle round trip poll/response time measurement from a single node canyield valuable user position information.

The command logic 176 is further coupled to the receiver AGC 180, thematched filter ring detector (RD) 198, the acquisition and trackingcircuitry 178, the transmit local oscillator (LO) 162 and the ringgenerator (RG) 138 to command various modes of operation.

The economic feasibility of a mobile telephone system is related to thenumber of users that can be supported. Two significant limits on thenumber of users supported are bandwidth utilization efficiency and powerefficiency. In regard to bandwidth utilization efficiency, in either theground based cellular or mobile satellite elements, radio frequencyspectrum allocation is a severely limited commodity. Measuresincorporated in the invention to maximize bandwidth utilizationefficiency include the use of code division multiple access (CDMA)technology which provides an important spectral utilization efficiencygain and higher spatial frequency reuse factor made possible by the useof smaller satellite antenna beams. In regard to power efficiency, whichis a major factor for the satellite-mobile links, the satellitetransmitter source power per user is minimized by the use offorward-error-correcting coding, which in turn is enabled by the aboveuse of spread spectrum code division multiple access (SS/CDMA)technology and by the use of relatively high antenna gain on thesatellite. CDMA and forward-error-correction coding are known to thoseskilled in the art and no further details are given here.

The issue of band width utilization efficiency will now be considered indetail. The major contribution of SS/CDMA to spectral efficiency isclosely related to the concept of cellular "cluster". In existingFrequency Division or Time division multiple access technology, a givenfrequency or time slot allocation must be protected from interferencefrom nearby cells by users on the same frequency sub-band. Depending onthe degree of protection required, it may be necessary to preclude thereuse of the cell "X" frequencies on a number of cells, N, surrounding"X". That number is called the "cluster size." Because each cell canthen utilize only one Nth of the total allocatable channels, it will beseen, all other things being equal, that the "frequency reuse factor"and spectral utilization efficiency are inversely proportional to thecluster size, N.

Field tests of the FM-frequency division multiplex ground cellularsystem, Macdonald, V. H., The Cellular Concept, Bell Systems TechnicalJournal, p. 15, January 1979, determined that a signal-to-interferenceratio of 17 dB or better is required for good to excellent quality to beperceived by most listeners. This, combined with progagation and fadingstudies, yielded the criterion that the separation between co-channelsites should be at least 6.0 times the maximum distance to a user withinthe cell using omni-directional antennas at the ground nodes. In orderto achieve this separation, the cluster size must be at least N=12 cellsper cluster. Thus one may use only 1/12 of the total allocatablecapacity per cell.

In satellite service, the minimum cell size is inversely proportional tothe satellite dish diameter. For a given maximum feasible dish diameter,the number of available channels is strictly limited by the clustersize. In the planned AMSC system, C. E. Agnew et al., The AMSC MobileSatellite System, Proceedings of the Mobile Satellite Conference, NASA,JPL, May 1988, the effective cluster size is 5, and one may use only 1/5or the total allocatable capacity per cell.

In a system in accordance with the invention, the cluster size is one.That is, each cell uses the same, full allocated frequency band. This ispossible because of the strong interference rejection properties ofspread spectrum code division multiple access technology (SS/CDMA). Theeffect of users in adjacent cells using the same band is qualitativelyno different than that of other users in the same cell, so may be takeninto account as an effective reduction in the number of users that canbe tolerated within a cell. The cumulative effect of all such other-cellinterferers may be calculated on the assumption of uniform density ofusers and a distance attenuation law appropriate to the case of groundpropagation or satellite beam pattern. Doing so, we find the multiplyingfactor for the ratio of total interference to in-cell origininterference of 1.4 for ground propagation and 2.0 for the satellitesystem. This factor may be accounted for as a multiplier equivalent ineffect to an effective cluster size for the CDMA system. Thus, finally,it is believed that in comparison with other systems we find frequencyreuse factor or bandwidth utilization efficiency factors inverselyproportional to effective cluster size in the ratios:

    0.71:0.5:0.2:0.08

for respectively the ground cellular component of the invention,satellite cellular component of the invention, the AMSC mobile satelliteconcept, and current ground cellular technology.

The second severely limited commodity in the satellite links issatellite prime power, a major component of the weight of acommunication satellite and thereby a major factor in satellite cost.Generally in systems such as this, the down links to individual usersare the largest power consumers and thus for a limited satellite sourcepower, may provide the limiting factor on the number of users that canbe served. Thus it is important to design the system for minimumrequired power per user. This requirement is addressed in the inventionin four ways. In the invention the system envisages the use of thehighest feasible satellite antenna gain. In one embodiment, power gainon the order of 45 dB and beamwidth of under one-degree are envisionedat L-band. This is accomplished by an antenna size of approximately 20meters. An antenna having a parabolic reflector with an offset feedlocated in the focal region of the reflector is used in one embodiment.The diameter of the rim of the reflector is approximately 20 meters andat S-band, a beamwidth of approximately 0.4 degrees results for each ofthe beams. As used herein, the expression "relatively narrow beamwidth"means a beamwidth resulting in a satellite cell at approximately 200 to500 km across.

Secondly, by virtue of the use of the spread spectrum technique, verylow rate high gain coding is available without penalty in terms ofincreased bandwidth occupancy.

Thirdly, the system utilizes channel bit interleaving/de-interleaving, akind of coded time diversity to provide power gain against deep fadingnulls. This makes it possible to operate at relatively low bit energy tonoise density ratio, on the order of 3 dB. This in turn reflects inminimum satellite power requirement per user.

Fourthly, two-way, adaptive power control as previously describedobviates the usual practice of continuously transmitting at a powerlevel which is 10 to 40 dB greater than required most of the time inorder to provide a margin for accommodating infrequent deep fades.

In addition to the above listed advantages, the Code Division Multiplexsystem has the following important advantages in the present system.Blank time when some of the channels are not in use reduces the averageinterference background. In other words, the system overloads andunderloads gracefully. The system inherently provides flexibility ofbase band rates; as opposed to FDM systems, signals having differentbaseband rates can be multiplexed together on an ad-hoc basis withoutcomplex preplanned and restrictive sub-band allocation plans. Not allusers need the same baseband rate. Satellite antenna sidelobe controlproblems are significantly reduced. The above mentioned numericalstudies of out-of-cell interference factors show that secondary loberesponses may effectively be ignored. Co-code reassignment (that isreuse of the same spreading code) is feasible with just one beamseparation. However, because there are effectively (i.e. includingphasing as a means of providing independent codes) an unlimited numberof channel codes, the requirements on space division are eased; there isno need to reuse the same channel access i.e., spreading code.

By virtue of the above discussed design factors the system in accordancewith the invention provides a flexible capability of providing thefollowing additional special services: high quality, high rate voice anddata service; facsimile (the standard group 3 as well as the high speedgroup 4); two way messaging, i.e. data interchange between mobileterminals at variable rates; automatic position determination andreporting to within several hundred feet; paging rural residentialtelephone; and private wireless exchange.

It is anticipated that the satellite will utilize geostationary orbitsbut is not restricted to such. The invention permits operating in otherorbits as well. The system network control center 12 is designed tonormally make the choice of which satellite or ground node a user willcommunicate with. In another embodiment, as an option, the user canrequest his choice between satellite link or direct ground based linkdepending on which provides clearer communications at the time orrequest his choice based on other communication requirements.

While a satellite node has been described above, it is not intended thatthis be the only means of providing above-ground service. In the casewhere a satellite has failed or is unable to provide the desired levelof service for other reasons, for example, the satellite has been jammedby a hostile entity, an aircraft or other super-surface vehicle may becommissioned to provide the satellite functions described above. The"surface" nodes described above may be located on the ground or in waterbodies on the surface of the earth. Additionally, while users have beenshown and described as being located in automobiles, other users mayexist. For example, a satellite may be a user of the system forcommunicating signals, just as a ship at sea may or a user on foot.

While several particular forms of the invention have been illustratedand described, it will be apparent that various modifications can bemade without departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited, except bythe appended claims.

What is claimed is:
 1. A cellular communications system comprising:atleast one space node having a multiple beam antenna positioned so toestablish a first set of cells, each space node including means fortransmitting and receiving different predetermined sets of code divisionmultiple access coded, spread spectrum waveforms digitally modulated andincorporating forward error correction coding with the waveforms beinglocated in a predetermined frequency band common to all space nodes; theantenna having a reflector with a multiple element feed disposed in thefocal region of the reflector; at least one surface node positioned soto establish a second set of cells, each surface node including meansfor transmitting and receiving the predetermined sets of code divisionmultiple access coded, spread spectrum waveforms in the predeterminedfrequency band; and a plurality of user units within the cells, eachunit including means for communicating with each satellite node and witheach surface node and being operatively responsive to a predeterminedone of the sets of code division multiple access coded waveforms tothereby establish selective communication with at least one of thenodes.
 2. A cellular communications system comprising:at least one spacenode having a multiple beam antenna positioned so to establish a firstset of cells, each space node including means for transmitting andreceiving different predetermined sets of code division multiple accesscoded, spread spectrum waveforms digitally modulated and incorporatingforward error correction coding with the waveforms being located in apredetermined frequency band common to all space nodes, wherein theantenna comprises a reflector with a multiple element feed disposed inthe focal region of the reflector; at least one surface node positionedso to establish a second set of cells, each surface node including meansfor transmitting and receiving the predetermined sets of code divisionmultiple access coded, spread spectrum waveforms in the predeterminedfrequency band; a plurality of user units within the cells, each unitincluding means for communicating with each satellite node and with eachsurface node and being operatively responsive to a predetermined one ofthe sets of code division multiple access coded waveforms to therebyestablish selective communication with at least one of the nodes; and anetwork controller operationally connected with each space node and witheach surface node to selectively allocate communications with said userunits among said space and surface nodes.
 3. The cellular communicationssystem as in any one of the preceding claims further comprising positionmeans for determining the position of a selected user unit by providinga timing signal to the user unit from one or more nodes, measuring theresponse time of the user unit to each timing signal, and determiningthe position of the user unit based on such measurements.
 4. Thecellular communications system as in claim 3 wherein the position meansis also for determining which cell a selected user unit is in and forindicating the location of the cell.
 5. The cellular communicationssystem as in claims 1 or 2 wherein the user unit comprises a hand-held,portable transceiver having a substantially non-directional antenna forcommunicating with the nodes.
 6. The communications system as in any oneof claims 1 through 2 wherein:a user unit and a node each comprises atransceiver for communicating with each other, each of the transceiverscomprising: a transmitter which outputs a transmitted signal at acontrollable power level and includes level data in said signal which isrepresentative of the power level of the transmitter; and a receiverwhich receives the transmitted signal including the level data from thetransmitter of the other transceiver, and which comprises:measurementmeans for measuring the signal strength of the signal received from theother transmitter; comparison means for comparing the measured signalstrength to the received level data; and means for controlling theoutput power level of the associated transmitter of the receiver in thetransceiver in accordance with said comparison.
 7. The cellularcommunications system of any of claims 1 through 3 wherein a satellitenode includes processing means for receiving signals in a firstpredetermined plurality of beams, for multiplexing the received signalstogether and for transmitting the multiplexed signals in a secondpredetermined plurality of beams.
 8. The cellular communications systemas in claims 1 or 2 wherein the multi-beam antenna of the space nodeprovides beams of relatively narrow beamwidth.
 9. The cellularcommunications system as in claim 8 wherein the beamwidth of at leastone of the beams of the antenna is less than one degree.
 10. Thecellular communications system as in claim 2 wherein the networkcontroller controls the system such that the user unit communicates withthe space node of the cell it is within or with the surface node of thecell it is within based on selectively considering the quality of thesignal received from the user unit at each node, the location of theparty with which the user unit desires to communicate, and apredetermined allocation of communication resources.
 11. The cellularcommunications system as in claims 1 or 2 wherein the predeterminedfrequency band is divided into a plurality of sub-bands andcommunications with each space node are conducted in a first sub-bandand communications with each surface node are conducted in a secondsub-band.
 12. A cellular communications system comprising:at least onespace node having a multiple beam antenna positioned so to establish afirst set of cells, each space node including means for transmitting andreceiving different predetermined sets of code division multiple accesscoded, spread spectrum waveforms digitally modulated and incorporatingforward error correction coding with the waveforms being located in apredetermined frequency band common to all space nodes; at least onesurface node positioned so to establish a second set of cells, eachsurface node including means for transmitting and receiving thepredetermined sets of code division multiple access coded, spreadspectrum waveforms in the predetermined frequency band; a plurality ofuser units within the cells, each unit including means for communicatingwith each satellite node and with each surface node and beingoperatively responsive to a predetermined one of the sets of codedivision multiple access coded waveforms to thereby establish selectivecommunication with at least one of the nodes; and a network controlleroperationally connected with each space node and with each surface nodeto selectively allocate communications with said user units among saidspace and surface nodes, said network controller controlling the systemsuch that the user unit communicates with the space node of the cell itis within or with the surface node of the cell it is within based onselectively considering the quality of the signal received from the userunit at each node, the location of the party with which the user unitdesires to communicate, and a predetermined allocation of communicationresources.